Current signal monitoring is particularly challenging. Although Hall effect sensors are closed-loop, more typically current measurement methods are open loop. Present methodologies used for current measurement fall into four main categories: resistive shunt, current transformer, Hall effect (Lorentz force), and giant magnetoresistive (GMR). Each of these approaches has practical difficulties and limitations. Table 1 summarizes some of the issues encountered for each of these approaches:
TABLE 1Current Measurement Challenges in the Space EnvironmentResistive Current ShuntTransformerHall EffectGMRNo electrical Cannot measure Requires temperatureRequires isolationDC current*dependent offsetcontrol of thecompensationgeometry*High acquisition Produces AC Requires external Amplification costinsertion losspower supplycircuitstage requiredInsertion lossOutput is Complex operationIssues with frequencycommondependentmode rejectionLarge size/Very large Limited thermal Nonlinear weight penaltysize/weightrange &outputfor any penalty for anylow thermal driftresponsemeasurementsignificant >100 Hzpower levelmeasurementRequires invasiveRequires High weight Noise insertion (lineEMI/RFIpenalty (upsusceptibilityresistance/isolation to 2000 Kg)power losspackagingAmplification Lack of stage requiredrobustnessDifficult to install*Zero-flux current transformer can measure to do but is designed for high power applications*Low currents detection normally requires that the observed current flow into trace on the GMR chip located immediately over the GMR (Wheatstone bridge) resistors.
There has been additional work on solid-state solutions such as magnetodiodes (highly nonlinear and temperature dependent) and magnetotransistors (higher levels of noise, nonlinearity, temperature dependent, large offset values); however, their performance has prevented any commercial development to date. There has also been interest to exploit the Faraday effect for development of optical fiber based magnetic field sensors, but these are both complex and expensive to acquire, tricky to install, require optoelectronic conversion and are susceptible to ‘latch-up in space environment.
For many applications installing a current sensor can becomes further challenging as the measurement solution may be required to exhibit high galvanic isolation, good accuracy, radiation effect tolerance, wide temperature operation and, critically, be capable of measuring both alternating current, even to very low frequencies, and measuring direct current. There is a need for a highly effective, compact, lightweight, low complexity current sensor, not subject to thermal effects, that can meet these needs.
One approach known to the present inventors is to exploit the properties of magnetostrictive materials. Magnetostrictive materials are materials that couple their magnetic and electric behaviors. In particular, the material will change shape when subjected to a magnetic field. Such materials include Terfenol and Galfenol. By placing a magnetostrictive element adjacent to a current carrying conduit the magnetic field caused by the flow of current will interact with the magnetostrictive material as to induce a strain (ΔL/L). FIG. 1 shows a device where a cantilevered beam 4 with rigid support 5, whose free end is placed proximal to a current carrying cable 1 and has a magnetostrictive element 2 attached thereto. A piezoelectric element 3 is attached to one side of magnetostrictive element 2. When current 19 flows, Maxwell's equations state that it will induce a magnetic field which will cause an axial strain to occur in the magnetostrictive material 2. This strain is then transferred to the coupled piezoelectric element that, being piezoelectric, creates an electric potential. This voltage is proportional to the strength of the current flow 19 being monitored; however, the current flow induced strain in the magnetostrictive element is extremely small. It is known that the direct-effect dielectric constants of piezoelectric materials is low, so coupling the strain of the magnetostrictive insert to the piezoelectric material substantially reduces an already very small signal to where it becomes negligible and has to resolve noise and other measurement disturbance issues.
There remains a need for magnetostrictive based current sensing devices that induce a strain of sufficient magnitude in the magnetostrictive element per ampere of current flow to allow for measurement of a statistically significant range of current. There is also a need for magnetostrictive based current sensing device that exhibits minimal signal loss in conversion of induced strain on the magnetostrictive element.